CN117806136A

CN117806136A - Optical isolation module

Info

Publication number: CN117806136A
Application number: CN202410142547.9A
Authority: CN
Inventors: 陶业争; D·J·W·布朗; A·A·沙夫甘斯; P·P·达斯
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2015-10-01
Filing date: 2016-09-30
Publication date: 2024-04-02
Also published as: JP2018533753A; TWI788814B; TWI724032B; KR102548316B1; TW202127151A; KR20180063232A; CN108351529A; JP6967509B2; US20230139746A1; US20200305263A1; JP2022023906A; TW201721234A; US10645789B2; US20180063935A1; JP2024003000A; US9832855B2; US11553582B2; KR20230098702A; US20170099721A1; JP7366979B2

Abstract

A light source for a lithographic tool comprising: a source configured to emit a first light beam and a second light beam, the first light beam having a first wavelength and the second light beam having a second wavelength, the first wavelength and the second wavelength being different; an amplifier configured to amplify the first and second light beams to generate first and second amplified light beams, respectively; and an optical isolator between the light source and the amplifier, the optical isolator comprising: a plurality of dichroic optical elements and an optical modulator between two dichroic optical elements.

Description

Optical isolation module

The application is a divisional application with the application number of 20160064410. X and the name of optical isolation module, which is submitted in the international application date 2016, 9, 30, 5, 3 and enters the national stage of China.

Cross Reference to Related Applications

The present application claims the benefit of U.S. provisional application No. 62/236,056 entitled "OPTICAL ISOLATION MODULE" filed on 1 month 10 of 2015 and the benefit of U.S. Ser. No. 14/970,402 entitled "OPTICAL ISOLATION MODULE" filed on 15 month 12 of 2015, which are incorporated herein by reference.

Technical Field

The present disclosure relates to an optical isolation module. The optical isolation module can be used in an Extreme Ultraviolet (EUV) light source.

Background

Extreme ultraviolet ("EUV") light (e.g., electromagnetic radiation having a wavelength of about 50nm or less (also sometimes referred to as soft x-rays) and including light having a wavelength of about 13 nm) may be used in lithographic processes to produce very small features in a substrate (e.g., a silicon wafer).

Methods of generating EUV light include, but are not necessarily limited to, converting materials having an element (e.g., xenon, lithium, or tin) with an emission line in the EUV range in a plasma state. In such methods, commonly referred to as laser produced plasma ("LPP"), the desired plasma may be produced by irradiating a target material, for example in the form of droplets, plates, ribbons, streams or clusters of material, with an amplified light beam, which may be referred to as a drive laser. For this process, the plasma is typically generated in a sealed container (e.g., a vacuum chamber) and monitored using various types of metrology equipment.

Disclosure of Invention

In one general aspect, a light source for a lithographic tool includes: a source configured to emit a first light beam and a second light beam, the first light beam having a first wavelength and the second light beam having a second wavelength, the first wavelength and the second wavelength being different; an amplifier configured to amplify the first and second light beams to generate first and second amplified light beams, respectively; and an optical isolator between the source and the amplifier, the optical isolator comprising: a plurality of dichroic optical elements and an optical modulator between two dichroic optical elements.

Implementations may include one or more of the following features. The optical modulator may comprise an acousto-optic modulator. Each of the dichroic optical elements may be configured to reflect light having a first wavelength and transmit light having a second wavelength; and the acousto-optic modulator may be positioned on the beam path between the two dichroic optical elements, the acousto-optic modulator may be positioned to receive reflected light from the two dichroic optical elements, the acousto-optic modulator may be configured to transmit the received light when the received light propagates in a first direction relative to the acousto-optic modulator and to deflect the received light from the beam path when the received light propagates in a second direction relative to the acousto-optic modulator, the second direction being different from the first direction. The first and second light beams may be pulsed light beams. The energy of the first amplified light beam may be less than the energy of the second amplified light beam. The first amplified light beam may have energy sufficient to deform target material in the target material droplets into a modified target, the modified target comprising target material in a geometric distribution that differs from a distribution of target material in the target material droplets, the target material comprising a material that Emits Ultraviolet (EUV) light when in a plasma state, and the second amplified light beam having energy sufficient to convert at least some of the target material in the modified target into a plasma that emits EUV light.

An acousto-optic modulator may be positioned on the beam path between the two dichroic optical elements and may be positioned to receive light reflected from the two dichroic optical elements, the acousto-optic modulator may be configured to receive a trigger signal, and the acousto-optic modulator may be configured to deflect light received from the beam path in response to receiving the trigger signal and to otherwise transmit the received light onto the beam path.

The light source may further comprise a second optical modulator between the source and the amplifier. The second optical modulator is between the two dichroic optical elements, and the second optical modulator is on a different beam path than the optical modulator.

The source may comprise a laser source. The source may comprise a plurality of sources, the first beam being generated by one of the sources and the second beam being generated by another of the sources. The source may include one or more preamplifiers.

In another general aspect, an apparatus for an Extreme Ultraviolet (EUV) light source includes: a plurality of dichroic optical elements, each of the dichroic optical elements configured to reflect light having a wavelength in a first wavelength band and transmit light having a wavelength in a second wavelength band; and an optical modulator located on the beam path between the two dichroic optical elements, the optical modulator being positioned to receive light reflected from the two dichroic optical elements, and the optical modulator being configured to transmit the received light when the received light propagates in a first direction on the beam path and to deviate the received light from the beam path when the received light propagates in a second direction on the beam path, the second direction being different from the first direction, wherein the first wavelength band comprises wavelengths of the pre-pulse light beam and the second wavelength band comprises wavelengths of the main beam.

Implementations may include one or more of the following features. The optical modulator may be an acousto-optic modulator. The device may further include a control system configured to provide a trigger signal to the acousto-optic modulator, and the acousto-optic modulator may be configured to deflect light away from the beam path and otherwise transmit light onto the beam path in response to receiving the trigger signal.

The device may further include a second optical modulator, wherein the second optical modulator is located between the two dichroic optical elements, and the second optical modulator is positioned to receive light transmitted by the two dichroic optical elements. The optical modulator and the second optical modulator may be located between the same two dichroic optical elements, and the second optical modulator may be located on a second beam path different from the beam path.

In another general aspect, a method includes: reflecting the first light beam at a first dichroic optical element, the reflected first light beam passing through an optical modulator and an amplifier to produce an amplified first light beam; transmitting the second light beam through the first dichroic optical element, the second dichroic optical element, and the amplifier to produce an amplified second light beam; receiving a reflection of the amplified first light beam at a second dichroic optical element, wherein an interaction between the reflection of the amplified first light beam and the second dichroic optical element directs the reflected amplified first light beam to an optical modulator; and deflecting the reflection of the amplified first light beam at the optical modulator, thereby directing the reflection of the amplified first light beam away from the source of the first light beam.

Implementations may include one or more of the following features. The trigger signal may be provided to the optical modulator after the first light beam passes through the optical modulator and before the amplified first light beam is reflected at the optical modulator.

The trigger signal may cause the optical modulator to be in a state in which the optical modulator deflects incident light.

The amplified first light beam may propagate toward the initial target area. The reflection of the first amplified light beam may be generated by an interaction between the first amplified light beam and the droplet of target material in the initial target area. The second amplified light beam may propagate toward the target region, and interactions between the target material and the second amplified light beam may produce reflections of the second amplified light beam, the method further comprising: the reflection of the second amplified light beam is transmitted through a second dichroic optical element, and the reflection of the second amplified light beam is deflected at a second optical modulator, thereby directing the reflection of the second amplified light beam away from the source of the second light beam. The source of the first light beam and the source of the second light beam may be the same source. The source of the first light beam may be a first optical subsystem in the source and the source of the second light beam may be a second optical subsystem in the source.

Embodiments of any of the techniques described above may include methods, processes, optical isolators, kits, or pre-assembly systems, or apparatus for retrofitting existing EUV light sources. The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

Drawings

Fig. 1 and 2 are block diagrams of exemplary optical systems.

Fig. 3 and 6 are block diagrams of exemplary optical isolators.

Fig. 4A and 4B are block diagrams of exemplary optical arrangements that may be used in the optical isolators of fig. 3 and 6.

Fig. 5A and 5B are timing diagrams associated with an exemplary optical modulator.

FIG. 7 is a block diagram of an exemplary control system.

Fig. 8A and 8B are block diagrams of a driving laser system for an Extreme Ultraviolet (EUV) light source.

Fig. 9, 10A, 10B, 11A, 11B, 12A-12C, and 13A-13C are examples of experimental data collected with and without an optical isolator.

Detailed Description

Referring to fig. 1, a block diagram of an exemplary optical system 100 is shown. The optical system 100 is part of an Extreme Ultraviolet (EUV) light source. The optical system 100 includes a light source 102 that generates a light beam 110. Light beam 110 emanates from light source 102 and light beam 110 propagates along path 112 in direction z toward target area 115.

Target region 115 receives target 120, target 120 comprising a material that emits EUV light when converted to plasma. Target 120 is reflective at one or more wavelengths of beam 110. Because target 120 is reflective, all or a portion of beam 110 may be reflected along path 112 in a direction other than the z-direction when beam 110 interacts with target 120. The reflected portion of beam 110 is labeled as reflection 113. Reflection 113 may travel on path 112 in a direction opposite to the z-direction and return into light source 102. The reflection (e.g., reflection 113) of the forward beam (the beam propagating from the light source 102 toward the target region 115) is referred to as "back reflection".

The light source 102 includes a light generation module 104, an optical isolator 106, and an optical amplifier 108. The light generation module 104 is a source of light (e.g., one or more lasers, lamps, or any combination of these elements). The optical amplifier 108 has a gain medium (not shown) on the beam path 112. When the gain medium is excited, the gain medium provides photons to beam 110, thereby amplifying beam 110 to produce amplified beam 110. The optical amplifier 108 may comprise more than one optical amplifier arranged with a corresponding gain medium on path 112. The optical amplifier 108 may be all or a portion of a drive laser system (e.g., drive laser system 880 of fig. 8B).

The light generation module 104 emits a light beam 110 onto a beam path 112 toward the optical isolator 106. Optical isolator 106 passes beam 110 in the z-direction and toward target region 115 to optical amplifier 108. However, the optical isolator 106 blocks the retro-reflection 113. Thus, and as discussed in more detail below, the optical isolator 106 prevents back reflection into the light generation module 104. By preventing back reflection into the light generation module 104, additional optical power may be transferred to the target 120, which may result in an increase in the amount of EUV light generated.

Referring to fig. 2, a block diagram of an EUV light source 200 including an exemplary light source 202 is shown. Light source 202 may be used in place of light source 102 in optical system 100 (fig. 1). The light source 202 comprises a light generating module 204, the light generating module 204 comprising two optical subsystems 204a, 204b, an optical amplifier 108 and an optical isolator 106. The optical isolator 106 is on the path 112 and between the optical amplifier 108 and the light generating module 204.

The optical subsystems 204a, 204b generate a first light beam 210a and a second light beam 210b, respectively. In the example of fig. 2, the first beam 210a is represented by a solid line and the second beam 210b is represented by a dashed line. The optical subsystems 204a, 204b may be, for example, two lasers. In FIG. 2 In an example, the optical subsystems 204a, 204b are two carbon dioxide (CO ₂ ) A laser. However, in other embodiments, the optical subsystems 204a, 204b are different types of lasers. For example, optical subsystem 204a may be a solid state laser and optical subsystem 204b may be a CO ₂ A laser.

The first light beam 210a and the second light beam 210b have different wavelengths. For example, two COs are included in the optical subsystems 204a, 204b ₂ In an embodiment of the laser, the wavelength of the first beam 210a may be about 10.26 micrometers (μm) and the wavelength of the second beam 210b may be between 10.18 μm and 10.26 μm. The wavelength of the second beam 210b may be about 10.59 μm. In these embodiments, even if two beams are generated from the same type of source, from the CO ₂ The different spectral lines of the lasers generate light beams 210a, 210b, resulting in light beams 210a, 210b having different wavelengths. The beams 210a, 210b may also have different energies.

The light generation module 204 further comprises a beam combiner 209, the beam combiner 209 directing a first light beam 210a and a second light beam 210b onto the beam path 112. Beam combiner 209 may be any optical element or collection of optical elements capable of directing first and second light beams 210a and 210b onto beam path 112. For example, beam combiner 209 may be a collection of mirrors, some of which are positioned to direct first beam 210a onto beam path 112 and others of which are positioned to direct second beam 210b onto beam path 112. The light generation module 204 may also include a pre-amplifier 207, the pre-amplifier 207 amplifying the first light beam 210a and the second light beam 210b within the light generation module 204.

The first and second light beams 210a, 210b may propagate on the path 112 at different times, but the first and second light beams 210a, 210b follow the path 112 and both light beams 210a, 210b pass through substantially the same spatial region up to the optical isolator 106 and through the optical amplifier 108. As shown in fig. 3 and 6, the first and second light beams 210a and 210b are separated within the optical isolator 106 and then propagate on path 112 to the optical amplifier 108.

The first light beam 210a and the second light beam 210b are angularly dispersed by the beam delivery system 225 such that the first light beam 210a is directed towards the initial target area 215a and the second light beam 210b is directed towards the modified target area 215b, the target area 215b being displaced in the-y direction relative to the initial target area 215 a. In some embodiments, the beam delivery system 225 also focuses the first and second light beams 210a, 210b to locations within or near the initial target region 215a and the modified target region 215b, respectively.

In the example shown in fig. 2, the initial target area 215a receives the initial target 220a and the first light beam 210a. The first beam 210a has energy sufficient to modify (or initiate spatial reconfiguration of) the geometric distribution of the target material in the initial target 220a to a modified target received in the modified target region 215 b. The second light beam 210b is also received in the modified target area 215 b. The second beam 210b has energy sufficient to convert at least some of the target material in the modified target 220b into a plasma that emits EUV light. In this example, the first light beam 210a may be referred to as a "pre-pulse" and the second light beam 210b may be referred to as a "main pulse".

The first light beam 210a may reflect off of the initial target 220a, producing a back reflection 213a, and the back reflection 213a may propagate along the path 112 in a direction other than the z-direction and into the optical amplifier 108. Because the first light beam 210a is used to modify the spatial characteristics of the initial target 220a and is not intended to convert the initial target 220a into a plasma that emits EUV light, the first light beam 210a has a lower energy than the second light beam 210 b. However, the reflection of the first beam 210a may have more energy than the reflection of the second beam 201 b.

The first light beam 210a (and the reflection 213 a) propagates through the optical amplifier 108 before the second light beam 210 b. Thus, when the reflection 213a passes through the gain medium of the optical amplifier 108, the gain medium of the optical amplifier 108 may still be excited. As a result, the reflection 213a may be amplified by the amplifier 108. Further, the initial target 220a may be substantially spherical, dense, and highly reflective, while the modified target 220b may be disk-like in shape (or other non-spherical shape), less dense, and less reflective. Due to the non-spherical shape, the modified target 220b may be positioned to reduce the amount of light reflected back onto the path 112 due to the interaction between the second light beam 210b and the modified target 220 b. For example, the modified target 220b may be tilted in the x-z and/or y-z plane relative to the direction of propagation of the light beam 210b, or the modified target 220b may be away from the focal point of the second light beam 210 b.

In some embodiments, the modified target 220b is not tilted in the x-z and/or y-z plane, and instead the modified target 220b is oriented such that the side of the modified target 220b having the largest spatial extent is in a plane perpendicular to the direction of propagation of the second light beam 210 b. Orienting the modified target 220b in this manner (which may be referred to as a "flat" target orientation) may enhance absorption of the second light beam 210 b. In some embodiments, such orientation may increase the absorption of the second light beam 210b by about 10% as compared to a case in which the modified target 220b is tilted 20 degrees (°) relative to a plane perpendicular to the direction of propagation of the second light beam 210 b. Positioning the modified target 220b in a flat orientation may increase the amount of reflected light propagating back into the light source 202. However, because the light source 202 includes the optical isolator 106, the modified target 220b may have a flat orientation because the optical isolator 106 is used to reduce the effects of reflections in the flat orientation that may be produced by the modified target 220 b.

Finally, because the second beam 210b has a relatively large energy, the forward propagation of the second beam 210b through the amplifier 108 saturates the gain medium, making it almost impossible to provide the energy that the amplifier 108 can provide to the back reflection of the second beam 210 b. Thus, even if the first light beam 210a has a lower energy than the second light beam 210b, the back reflection 213a produced by the first light beam 210a may be substantial and may be greater than the back reflection produced by the second light beam 210 b.

As described below, the optical isolator 106 prevents back reflections generated from the first light beam 210a from entering the light generation module 204. The optical isolator 106 may also prevent back reflection from the second light beam 210b from entering the light generation module 204, and one example of such an embodiment is shown in fig. 6. Because the optical isolator 106 prevents potentially damaging back reflections from reaching the light generation module 204, a higher energy beam may be generated from the light generation module 204, resulting in more energy being delivered to the modified target 220b and more EUV light being produced. In some embodiments, the average amount of EUV light produced may be increased by about 20% by using an optical isolator 106.

Referring to FIG. 3, a block diagram of an exemplary optical isolator 306 is shown. The optical isolator 306 may be used as the optical isolator 106 in the light source 102 (fig. 1), the light source 202 (fig. 2), or any other light source. The optical isolator 306 is discussed with respect to the light source 202.

Optical isolator 306 includes a dichroic optical element 331, a reflective element 332, an optical modulator 335, and a dichroic element 336. The optical isolator 306 may also include optical arrangements 333, 334. Dichroic elements 331 and 336 are on beam path 112. Dichroic elements 331 and 336 may be any optical component capable of separating or filtering light according to its wavelength. For example, dichroic elements 331 and 336 may be dichroic mirrors, dichroic filters, dichroic beam splitters, or combinations of these elements. The dichroic elements 331 and 336 may be identical to each other, or they may have different configurations. In the example of fig. 3, dichroic elements 331 and 336 reflect one or more wavelengths of first light beam 210a and transmit one or more wavelengths of second light beam 210 b.

The first light beam 210a is reflected from the dichroic element 331 onto a beam path 314, the beam path 314 being located between the dichroic elements 331 and 336 and having a spatial extent and form defined by a reflective element 332. Beam path 314 is different from beam path 112. Thus, in the optical isolator 306, the first light beam 210a does not remain on the beam path 112, and the first light beam 210a and the second light beam 210b are spatially separated from each other. Before reaching the dichroic element 336, the first light beam 210a propagates through the optical arrangements 333, 334 and the optical modulator 335 on the beam path 314, the dichroic element 336 reflecting the light beam 210a back onto the beam path 112. The second light beam 210b passes through dichroic element 331 and through dichroic element 336 while propagating through optical isolator 306 while remaining on beam path 112.

An optical modulator 335 is located in the beam path 314 between the dichroic elements 331 and 336. The optical modulator 335 is an optical element capable of deflecting incident light off of the path 314. The optical modulator 335 may be adjusted between an open state and a closed state such that the optical modulator 335 may transmit the first light beam 210a and block the reflection 213a (reflection of the first light beam 210a from the initial target 220 a).

The optical modulator 335 may be, for example, an acousto-optic modulator (AOM). An acousto-optic modulator includes a medium (e.g., quartz or glass) connected to a transducer (e.g., a piezoelectric transducer). The movement of the transducer causes sound waves to form in the medium, creating a spatially varying refractive index in the medium. When the medium comprises sound waves, light incident on the medium is deflected. When acoustic waves are not present in the medium, the acousto-optic modulator transmits incident light without deflection. Other optical modulators may be used as modulator 335. For example, the optical modulator 335 may be a faraday rotator or an electro-optic modulator (EOM). The modulator 335 may be a combination of such devices and may include more than one device of the same type.

In embodiments where the optical modulator 335 is an acousto-optic modulator, the transducer is moved when the reflection 213a is expected to enter the path 314. At other times, the transducer does not move or vibrate. Thus, beam 210a (the forward "pre-pulse") passes through optical modulator 335, remains on path 314, and eventually rejoins path 112. However, the reflection 213a is deflected (shown as deflection 217a in fig. 3) away from the path 314. As a result, the reflection 213a does not reach the light generation module 204 (fig. 2).

Because the optical modulator 335 may be configured to transmit incident light only at a particular time, the optical isolator 306 provides a time gate-based isolation technique (rather than a polarization-based technique). Additionally, the optical isolator 306 may be used in combination with polarization-based isolation techniques. For example, the polarization of the back-reflected light beam may be different from the polarization of the forward light beams 210a, 210b, and a polarization isolator 303 including a polarizing element (e.g., a thin film polarizer) may be placed between the optical isolator 306 and the optical amplifier 108 (fig. 1 and 2) to provide additional back-reflection blocking. The polarizing element of polarizing isolator 303 may be configured to primarily suppress reflection of second light beam 210b, thereby allowing optical isolator 306 to be adapted to suppress reflection of first light beam 210 a. By using different techniques to suppress the reflection of the first and second light beams 210a, 210b, the total amount of reflection from any source to the light generation module 204 may be reduced.

In some embodiments, the optical isolator 306 includes a first optical arrangement 333 and a second optical arrangement 334. The first light beam 210a passes through a first optical arrangement 333 before reaching the optical modulator 335. The first optical arrangement 333 may be any optical element or collection of optical elements that reduces the beam diameter of the first light beam 210 a. After passing through the optical modulator 335, the first light beam 210a passes through the second optical arrangement 334. The second optical arrangement 334 may be any optical element or collection of optical elements that expands the beam diameter of the second light beam 210 b. The speed at which the optical modulator 335 can switch between open (state in which incident light is transmitted by the optical modulator 335) or closed (state in which incident light is deflected or blocked by the optical modulator 335) increases as the beam diameter decreases. Thus, by reducing the diameter of the first light beam 210a, the first optical arrangement 333 allows the optical modulator 335 to switch between open and closed more quickly than embodiments lacking the first optical arrangement 333, and vice versa. In some embodiments, the beam diameter of the light beam 210a may be reduced to about 3 millimeters (mm).

The second optical arrangement 334 expands the diameter of the first light beam 210a before directing the first light beam 210a onto the path 112. Additionally, the second optical arrangement 334 reduces the beam diameter of the reflection 213a before the reflection 213a reaches the optical modulator 335. By reducing the beam diameter of the reflection 213a, the optical modulator 335 must switch between an open state and a closed state to block the speed reduction of the reflection 213 a.

Referring to fig. 4A and 4B, block diagrams of exemplary optical arrangements 433 and 434, respectively, are shown. The optical arrangements 433, 434 may be used as the optical arrangements 333, 334, respectively, in the optical isolator 306 (fig. 3). The optical arrangements 433, 434 are galilean telescopes with one convex lens and one concave lens. In the optical arrangement 433, a concave lens 442 is located between the convex lens 441 and the optical modulator 335. In the optical arrangement 434, a concave lens 443 is located between the optical modulator 335 and a convex lens 444. Both arrangements 433, 434 reduce the diameter of the light beam propagating towards the optical modulator 335. When the optical arrangements 433, 434 are used together in the configuration shown in fig. 3, the beam diameter of the light beam 210a is reduced before being incident on the optical modulator 335, and after passing through the optical modulator 335, the beam diameter of the light beam 210a is enlarged by the optical arrangement 434. The beam diameter of the reflection 213a is reduced by the optical arrangement 434 before reaching the optical modulator 335. The reflection 213a does not pass through the optical arrangement 433 because the optical modulator 335 deflects the reflection 213a from the beam path 314.

The optical arrangements 433 and 434 may be the same galilean telescope or the arrangements 433 and 434 may comprise lenses with different characteristics (e.g. different focal lengths).

Referring to fig. 5A, an exemplary graph of the state of the optical modulator 335 as a function of time is shown. Fig. 5B shows the relative positions of the pulses of beam 510a and reflection 513a on the same time axis as shown in fig. 5. When system 200 is configured to use optical isolator 306 (fig. 3) as optical isolator 106, pulse 510a is a pulse of a light beam propagating through system 200 (fig. 2), and reflection 513a is a reflection of pulse 513a from initial target 220 a. Pulse 510a is a pulse of a pulsed light beam (used as a "pre-pulse" to form initial target 220 a).

From time t1 to time t2, the optical modulator 335 is closed (deflecting light from path 314 or otherwise preventing incident light from remaining on path 314). At time t2, the optical modulator 335 begins to transition to the off state. The optical modulator 335 is turned off between times t2 and t3, and during this time range, the optical modulator 335 transmits incident light. The optical modulator 335 transitions to closed at time t3 and is again closed at time t 4. As discussed above, the transition time (time between times t2 and t3 and time between t3 and t 4) may be reduced by reducing the beam diameter of the light being gated by the optical modulator 335.

Referring also to fig. 5B, times t2 and t3 are selected such that pulse 510a is incident on optical modulator 335 when modulator 335 is off. Thus, the pulse 510a passes through the optical modulator 335 to the initial target 220a. Times t3 and t4 are selected such that optical modulator 335 starts to close after transmission pulse 510a and closes when reflection 513a is incident on optical modulator 335. In this way, the optical modulator 335 provides time gate based isolation of the pre-pulse reflection 513 a.

In some embodiments, the beam diameters of pre-pulse 510a and reflection 513a may be 3mm. In embodiments where the optical modulator 335 is an acousto-optic modulator, the time for the optical modulator to switch from off to on and from on to off is determined by the beam diameter of the incident light and the speed of the sound in the material of the optical modulator. The material may be germanium (Ge) having an acoustic wave velocity of 5500 meters per second (m/s), for example. In this example, the transition time (the time for the optical modulator to transition from closed to open) is 375 nanoseconds (ns). The delay between the pre-pulse 510a and the reflection 513a may be, for example, 400ns. Thus, the pre-pulse 510a is transmitted by the optical modulator 335 and deflects the reflection 513a away from the path 314.

In some implementations, the optical modulator 335 is closed except for the period of the desired pulse 510 a. By remaining closed at other times, the optical modulator 335 prevents the reflection 513a from entering the light generating module 204. Additionally, by remaining closed, modulator 335 also prevents or reduces the effects of secondary reflections of pulse 510 a. Elements such as filters, pinholes, lenses, tubes, etc. on path 112 are sources of flicker and reflect incident light. These elements may reflect pulse 510b and cause secondary reflections that propagate on paths 112 and 314, and these secondary reflections are reflections other than reflection 513 a. In addition to when pulse 510a is incident on modulator 335, by keeping modulator 335 closed, secondary reflections are also prevented from entering light generation module 204. In addition, secondary reflections are removed from path 314 and are thus prevented from propagating back onto path 112. In this way, the secondary reflection cannot reach the initial target region 215a, the modified target region 215b, or the region between the regions 215a and 215 b. If the secondary reflections are able to reach these areas, the reflections may damage the target by separating the target before it reaches the modified target area 215 b. The secondary reflection may be referred to as a forward pulse excited by a reverse pulse (FER). The optical isolator 306 can help mitigate self-excitation that can limit the maximum of optical power delivered to the target region 215 b.

Referring to fig. 6, a block diagram of another exemplary optical isolator 606 is shown. An optical isolator 606 may be used in place of the optical isolator 106 in the system 100 (fig. 1) or the system 200 (fig. 2). Additionally, the optical isolator 606 can be used in any other optical system where it is desirable to prevent back reflection. The optical isolator 606 is discussed with respect to a configuration in which the optical isolator 606 is used as the optical isolator 106 in the system 200 (fig. 2). The optical isolator 606 may be used with the polarization isolator 303 discussed above with respect to fig. 3. In embodiments that include a polarization isolator 303, the polarization isolator 303 is positioned between the optical isolator 606 and the optical amplifier 108 (fig. 1 and 2) to provide additional blocking of the back reflection.

Optical isolator 606 is similar to optical isolator 306 (fig. 3) except that optical isolator 606 includes a second optical modulator 637. A second optical modulator 637 is located on path 112 and is positioned between dichroic optical element 331 and dichroic optical element 336. Similar to the optical modulator 335, the second optical modulator 637 transmits incident light when in an open state and deflects or blocks incident light when in a closed state. The second light beam 210b emanates from the light generation module 204 and propagates on path 112 to the dichroic optical element 331.

As described above, the dichroic optical element 331 transmits the wavelength of the second light beam 210 b. Thus, the second light beam 210b passes through the dichroic optical element 331 and is incident on the second optical modulator 637. When the second light beam 210b is incident on the modulator 637, the second optical modulator 637 is controlled to be in an off state and the second light beam 210b passes through the modulator 637 and the dichroic optical element 336, remains on the path 112 and reaches the modified target area 215b (fig. 2). A portion of the second light beam 210b reflects from the modified target 220b (except for converting at least some of the target material into a plasma that emits EUV light) and may propagate as reflection 213b along path 112 in a direction other than the z-direction.

Reflection 213b is transmitted by dichroic optical element 336 and is maintained on path 112. When the reflection 213b is incident on the modulator 637, the optical modulator 637 is closed and the reflection 213b is deflected from the path 112 into deflected light 217b. Thus, the second modulator 637 prevents the reflection 213b from reaching the light generation module 204 or reduces the amount of reflection 213b reaching the light generation module 204, thereby reducing or eliminating self-excitation from the light generation module 404 and allowing the second light beam 210b to have more energy. In some embodiments, the optical modulator 637 deflects 30-40% of the reflection 213 b. The time that the optical modulator 637 is off may be reduced to further reduce the amount of self-excitation. For example, reducing the off time from 20 microseconds (μs) to 2 μs may reduce self-excitation by 90%.

The second modulator 637 is closed except for the desired period of time for which the light beam 210b is being directed. By remaining closed at other times, the second modulator 637 prevents the reflection 213b from entering the light generation module 204. Additionally, by remaining closed, the second modulator 637 also prevents or reduces the effects of secondary reflections from the second light beam 210 b. On path 112, elements such as filters, pinholes, lenses, tubes, etc., are sources of flicker and reflect incident light. These elements may reflect the second light beam 210b and cause secondary reflections other than the reflection 213b (caused by the interaction between the second light beam 210b and the modified target 220 b). By keeping modulator 637 closed, secondary reflections are prevented from entering light generation module 204 and removed from path 112 except when second light beam 210b is incident on modulator 637.

The second optical modulator 637 may be the same as the modulator 335, or the second optical modulator 637 and the modulator 335 may be different types of modulators.

Referring to fig. 7, a block diagram of a system 700 is shown. The system 700 includes a light generation module 704, a control system 740, and an optical modulator 735. The light generation module 704 may be the light generation module 104 (fig. 1), the light generation module 204 (fig. 2), or any other system that generates light beams having different wavelengths. The optical modulator 735 may be the optical modulator 335 (fig. 3) and/or the optical modulator 637 (fig. 6).

Control system 740 provides a trigger signal 747 to optical modulator 735. The trigger signal 747 is sufficient to cause the optical modulator 735 to change state or begin to change state. For example, in embodiments where the optical modulator 735 is an acousto-optic modulator, the trigger signal 747 may transition the modulator to the closed state RRR by vibrating the transducer to form an acoustic wave in the modulator. The control system 740 may also receive data from the light generation module 704 via a signal 741 and may provide data to the light generation module 704 via a signal 742. In addition, control system 740 can also receive data from optical module 735 via signal 742.

Control system 740 includes electronic storage 743, electronic processor 744, and input/output (I/O) interface 745. The electronic processor 744 comprises one or more processors suitable for executing a computer program (e.g., a general purpose or special purpose microprocessor and any one or more processors of any type of digital computer). Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The electronic processor 744 may be any type of electronic processor.

The electronic storage 743 may be volatile memory (e.g., RAM) or non-volatile memory. In some embodiments, the electronic storage 743 may include non-volatile and volatile portions or components. The electronic storage 743 may store data and information used in the operation of the optical modulator 735. For example, electronic storage 743 may store timing information specifying when first beam 210a and second beam 210b are expected to propagate through system 200 (fig. 2). Electronic storage 743 may also store instructions (perhaps as a computer program) that when executed cause processor 744 to communicate with other components in control system 740, light generation module 704, and/or optical modulator 735. For example, the instructions may be instructions that cause the electronic processor 744 to provide a trigger signal 747 to the optical modulator 735 at a particular time specified by timing information stored on the electronic storage 743.

I/O interface 745 is any type of electronic interface that allows control system 740 to receive data and signals and/or provide data and signals to an operator, light generation module 704, optical modulator 735, and/or an automated process running on another electronic device. For example, the I/O interface 745 may include one or more of a visual display, a keyboard, or a communications interface.

Referring to fig. 8A, an LPP EUV light source 800 is shown. The optical systems 100 and 200 may be part of an EUV light source such as source 800. The LPP EUV light source 800 is formed by illuminating the target mixture 814 with an amplified light beam 810 at a target location 805, the amplified light beam 810 traveling along a beam path toward the target mixture 814. The target location 805 (also referred to as an irradiation site) is located within an interior 807 of the vacuum chamber 830. When the amplified light beam 810 impinges on the target mixture 814, the target material within the target mixture 814 is converted to a plasma state of elements having emission lines in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 814. These characteristics may include the wavelength of EUV light generated by the plasma, as well as the type and amount of debris released from the plasma.

The light source 800 also includes a target material delivery system 825, the target material delivery system 825 delivering, controlling, and directing a target mixture 814 in the form of droplets, a liquid stream, solid particles or clusters, solid particles contained within droplets, or solid particles contained within a liquid stream. The target mixture 814 includes a target material (e.g., water, tin, lithium, xenon, or any material that has an emission line in the EUV range when converted to a plasma state). For example, elemental tin may be used as pure tin (Sn); tin compounds (e.g. SnBr ₄ 、SnBr ₂ 、SnH ₄ ) The method comprises the steps of carrying out a first treatment on the surface of the Tin alloys (e.g., tin-gallium alloys, tin-indium-gallium alloys, or any combination of these alloys). The target mixture 814 may also include impurities such as non-target particles. Thus, in the absence of impuritiesIn the case of mass, the target mixture 814 is composed of only the target material. The target material delivery system 825 delivers the target mixture 814 into the interior 607 of the chamber 630 and to the target location 605.

Light source 800 includes a driving laser system 815, which driving laser system 815 produces an amplified light beam 810 due to population inversion within one or more gain media of laser system 815. The light source 800 includes a beam delivery system between the laser system 815 and the target location 805, which includes a beam delivery system 820 and a focusing assembly 822. Beam-delivery system 820 receives amplified light beam 810 from laser system 815 and diverts and modifies amplified light beam 810 as needed, and outputs amplified light beam 810 to focusing assembly 822. Focusing component 822 receives amplified light beam 810 and focuses light beam 810 to target location 805.

In some embodiments, the laser system 815 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses (and in some cases, one or more pre-pulses). Each optical amplifier includes a gain medium capable of optically amplifying a desired wavelength with a high gain, an excitation source, and internal optics. The optical amplifier may or may not have a laser mirror or other feedback device that forms the laser cavity. Thus, even without a laser cavity, laser system 815 would produce amplified light beam 810 due to population inversion in the gain medium of the laser amplifier. Furthermore, if a laser cavity is present to provide sufficient feedback to laser system 815, laser system 815 may generate amplified light beam 810 as a coherent laser beam. The term "amplified light beam" encompasses one or more of the following: light from the laser system 815 that is only amplified but not necessarily coherent laser oscillation; and light from the laser system 815 that is amplified and also oscillated by the coherent laser.

The optical amplifier in the laser system 815 may include a fill gas as a gain medium, the fill gas including CO ₂ And light having a wavelength between about 9100nm and about 11000nm (and in particular having about 10600 nm) may be amplified with a gain greater than or equal to 800.Amplifiers and lasers suitable for use in laser system 815 may include pulsed laser devices (e.g., pulsed gas discharge CO ₂ Laser device), pulsed laser devices operate at relatively high power (e.g., 10kW or higher) and high pulse repetition rates (e.g., 40kHz or higher), such as using DC or RF excitation to produce radiation at about 9300nm or about 10600 nm. The optical amplifier in the laser system 815 may also include a cooling system, such as water, that may be used when operating the laser system 815 at higher power.

Fig. 8B shows a block diagram of an example drive laser system 880. Drive laser system 880 may be used as part of drive laser system 815 in source 800. The drive laser system 880 includes three power amplifiers 881, 882 and 883. Any or all of the power amplifiers 881, 882, and 883 may include internal optical elements (not shown).

Light 884 exits the power amplifier 881 through an output window 885 and reflects off a curved mirror 886. After reflection, the light 884 passes through a spatial filter 887, reflects off a curved mirror 888, and enters a power amplifier 882 through an input window 889. The light 884 is amplified in the power amplifier 882 and directed out of the power amplifier 882 as light 891 through an output window 890. Light 891 is directed by folding mirror 892 toward amplifier 883 and enters amplifier 883 through input window 893. The amplifier 883 amplifies the light 891 and directs the light 891 out of the amplifier 883 as an output beam 895 through an output window 894. Folding mirror 896 directs output beam 895 upward (off the page) and toward beam delivery system 820 (fig. 8A).

Referring again to fig. 8B, the spatial filter 887 defines an aperture 897, and the aperture 897 may be, for example, a circle having a diameter between about 2.2mm and 3 mm. Curved mirrors 886 and 888 can be off-axis parabolic mirrors, for example, having focal lengths of about 1.7m and 2.3m, respectively. The spatial filter 887 may be positioned such that the aperture 897 coincides with the focal point of the drive laser system 880.

Referring again to fig. 8A, light source 800 includes a collector mirror 835 with an aperture 840 to allow amplified light beam 810 to pass through and reach target location 805. The collector mirror 835 may be, for example, an ellipsoidal mirror having a primary focus at the target position 805 and a secondary focus (also referred to as an intermediate focus) at the intermediate position 845, wherein EUV light may be output from the light source 800 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 800 can also include an open-ended hollow conical shield 850 (e.g., a gas cone) that tapers from the collector mirror 835 toward the target location 805 to reduce the amount of plasma-generated debris entering the focusing assembly 822 and/or the beam delivery system 820 while allowing the amplified light beam 810 to reach the target location 805. To this end, an air flow directed toward the target location 805 may be provided in the shroud.

Light source 800 may also include a master controller 855 coupled to droplet position detection feedback system 856, laser control system 857, and beam control system 858. Light source 800 may include one or more targets or droplet imagers 860, with targets or droplet imagers 860 providing an output indicative of the position of a droplet, e.g., relative to target position 805, and providing the output to a droplet position detection feedback system 856, which droplet position detection feedback system 856 may, e.g., calculate droplet position and trajectory from which droplet position errors may be calculated on a droplet by droplet basis or on average. The droplet position detection feedback system 856 thus provides droplet position errors as an input to the main controller 855. Thus, the master controller 855 can provide laser position, direction, and timing correction signals to, for example, the laser control system 857, and the laser control system 857 can be used, for example, to control laser timing circuitry and/or beam control system 858 to control the position of the amplified beam and the shape of the beam delivery system 820 to vary the position and/or focus power of the beam focus within the chamber 830.

Target material delivery system 825 includes a target material delivery control system 826, and target material delivery control system 826 is operable in response to signals from master controller 855, for example, to modify the release point of a droplet released by target material supply 827 to correct errors in the droplet reaching desired target location 805.

Additionally, the light source 800 may include light source detectors 865 and 870, the light source detectors 865 and 870 measuring one or more EUV light parameters including, but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular wavelength band, energy outside of a particular wavelength band, and angular distribution of EUV intensity and/or average power. The light source detector 865 generates a feedback signal that is used by the main controller 855. For example, the feedback signal may indicate errors in parameters such as timing and focusing of the laser pulses to correctly intercept the droplet at the correct location and time, thereby achieving effective and efficient EUV light generation.

Light source 800 may also include a guiding laser 875, and guiding laser 875 may be used to aim portions of light source 800 or to assist in guiding amplified light beam 810 to target location 805. In combination with the guiding laser 875, the light source 800 includes a metrology system 824, the metrology system 824 being positioned within the focusing assembly 822 to sample a portion of the light from the guiding laser 875 and the amplified light beam 810. In other embodiments, the metrology system 824 is placed within the beam delivery system 820. The metrology system 824 may include optical elements that sample or redirect a subset of light, such optical elements being made of any material capable of withstanding the power of the directed laser beam and the amplified light beam 810. The beam analysis system is formed by the metrology system 824 and the master controller 855, as the master controller 855 analyzes the sampled light from the guiding laser 875 and uses this information through the beam control system 858 to adjust the components within the focusing assembly 822.

Thus, in summary, the light source 800 produces an amplified light beam 810, which amplified light beam 810 is directed along a beam path to illuminate a target mixture 814 at a target location 805 to convert target material within the mixture 814 into a plasma that emits light in the EUV range. Amplified light beam 810 operates at a particular wavelength (also referred to as the drive laser wavelength) that is determined based on the design and properties of laser system 815. Additionally, the amplified light beam 810 can be a laser beam when the target material provides sufficient feedback into the laser system 815 to produce coherent laser light, or where the driving laser system 815 includes suitable optical feedback to form a laser cavity.

Referring to fig. 9, a graph 900 of example test data for an optical isolator such as optical isolator 306 (fig. 3) is shown. Graph 900 shows the measured power of a reverse pre-pulsed light beam as a function of time with the optical isolator in the on and off states. The reverse pre-pulse beam may be a beam such as the reflection 213a (fig. 2), the reflection 213a resulting from the interaction between the first beam 210a (fig. 2) and the initial target 220a (fig. 2) as discussed above. In the on state, the optical isolator blocks or reduces the effect of the reflection 213a by deflecting all or part of the reflection 213a from the beam path 314 such that the reflection 213a reaching the light generating module 204 is reduced or eliminated. In the on state, the optical isolator may operate, for example, as discussed with respect to fig. 5A and 5B. In the off state, the optical isolator is not activated and the system operates as if the optical isolator were not present.

In the example of fig. 9, the optical isolator is in an off state between times 905 and 910, and is otherwise in an on state. When the optical isolator is in the on state, the power to the reflection 213a of the light generation module 204 is very low and approaches zero watts (W). For example, the power to the reflection 213a of the light generation module 204 may be about or less than 0.1W. As described above, it is desirable to reduce the power reaching the reflection 213a of the light generation module 204. Conversely, when the optical isolator is in the off state, the power of the reflection 213a to the light generation module 204 is greater than 0 and may be between about 4.2W and 18.2W. Furthermore, when the optical isolator is in the off state, the power of the reflection 213a reaching the light generating module 204 varies greatly, which may cause instability of the system. Thus, in addition to reducing the amount of power in the reflection 213a, the optical isolator also reduces the variation in reflected power, resulting in a more stable system.

Referring to fig. 10A and 10B, additional example test data is shown. Fig. 10A shows the energy of EUV light generated as a function of the number of pulses when no optical isolator (e.g., optical isolator 306) is present in the system, and fig. 10B shows the energy of EUV light generated as a function of the number of pulses when an optical isolator is present in the system. When the optical isolator is not present, the average energy of EUV light is 3.4 millijoules (mJ). When the optical isolator is present, the average EUV energy increases to 4.1mJ.

Referring also to fig. 11A and 11B, the EUV light produced is also more stable when an optical isolator is present in the system. Fig. 11A shows a distribution of specific values of energy of EUV light generated when the optical isolator is not present, and fig. 11B shows a distribution of specific values of energy of EUV light generated when the optical isolator is present. The distribution of energy values of fig. 11B (when an optical isolator is used) shows that higher energy values occur more frequently than a system that does not employ an optical isolator, and all energy values are contained within a smaller range. Thus, the use of an optical isolator (e.g., optical isolator 306) results in EUV light having higher energy and also results in more stable (less varying) EUV light.

Referring to fig. 12A-12C and fig. 13A-13C, additional example test data is shown. Fig. 12A-12C show a target 1200 in a system lacking an optical isolator such as optical isolator 306 three times, and fig. 13A-13C show a target 1300 in a system that includes an optical isolator such as optical isolator 306 three times. The targets 1200 and 1300 include target materials that emit EUV light when in a plasma state. Targets 1200 and 1300 are shown coincident with targets 1200 and 1300 being in a position to receive a pre-pulse (e.g., initial target region 215a of fig. 2) and a position to receive a main pulse (e.g., modified target region 215b of fig. 2).

As discussed above with respect to fig. 5A and 5B, the optical isolator may reduce or eliminate secondary reflections from objects such as pinholes, lenses, tubes, optical elements, and the like. When a secondary reflection is present, the secondary reflection may reach the target as it moves from the initial target area 215a to the modified target area 215 b. Fig. 12A-12C illustrate one example of secondary reflections that interact with the target 1200 over time. As shown in fig. 12B and 12C, the object 1200 is spatially spread and separated over time as compared to fig. 12A. Fig. 13A-13C illustrate one example of a system that uses an optical isolator (e.g., optical isolator 306) to reduce or eliminate secondary reflections. The target 1300 (fig. 13A-13C) has a cleaner spatial profile than the target 1200 (fig. 12A-12C), which may result in an increased absorption of the incident beam and more target material available to interact with the second beam 210b (and thus produce more EUV light). Additionally, because the target 1300 is used with a light source that includes an optical isolator, the target 1300 may have a flat orientation relative to the direction of propagation of the incident light beam while still reducing or eliminating the effects of back-reflections and secondary reflections on the light source.

Other embodiments are within the scope of the following claims.

In embodiments where the optical subsystems 204a, 204b (fig. 2) are different types of optical subsystems, the optical subsystem 204a may be a rare-earth doped solid state laser (e.g., nd: YAG or erbium doped fiber (Er: glass)), and the wavelength of the first light beam 210a may be 1.06 μm. The optical subsystem 204b may be CO ₂ The laser and the wavelength of the beam 210b may be, for example, 10.26 μm. In these embodiments, the first and second light beams 210a, 210b may be amplified in separate optical amplifiers and may follow separate paths through the system 200. Furthermore, two separate optical isolators may be used, one for the first beam 210a and its corresponding reflection and the other for the beam 210b and its corresponding reflection.

The preamplifier 207 (fig. 2) may have a plurality of stages. In other words, the pre-amplifier 207 may include more than one amplifier in series and placed on the path 112.

Beams 110, 210a, and 210b may be pulsed beams. The power of the pulse (or pulse 510 a) of the first beam 210a may be, for example, 20-40 watts (W). The power of the pulses of the second beam 210b may be, for example, 300-500W.

The first beam 210a may be any type of radiation that may be directed at the initial target 220a to form a modified target 220 b. For example, the first beam 210 may be a pulsed beam generated by a laser. The first light beam 210 may have a wavelength of about 1 μm to about 10.6 μm. The duration of the pulses of the first light beam 210a may be, for example, 20-70 nanoseconds (ns), less than 1ns, 300 picoseconds (ps), between 100-300ps, between 10-50ps, or between 10-100 ps. The energy of the pulses of the first beam 210a may be, for example, 15-60 millijoules (mJ). When the pulse of the first beam 210a has a duration of 1ns or less, the energy of the pulse may be 2mJ. The time between the pulses of the first light beam 210a and the pulses of the second light beam 210b may be, for example, 1 to 3 microseconds (μs).

The initial target 220a and the target 115 may have any of the characteristics of the target mixture 814. For example, the initial target 220a and the target 115 may include tin.

The optical systems 100 and 200 may include a polarization isolator 303. In these embodiments of optical system 100, polarization isolator 303 is located between optical isolator 106 and optical amplifier 108.

Claims

1. An apparatus for an Extreme Ultraviolet (EUV) light source, the apparatus comprising:

A plurality of dichroic optical elements, each of the dichroic optical elements configured to reflect light having a wavelength in a first wavelength band and transmit light having a wavelength in a second wavelength band; and

an optical modulator located on a beam path between two of the dichroic optical elements, the optical modulator positioned to receive light reflected from the two dichroic optical elements and configured to transmit the received light when the received light propagates in a first direction on the beam path and deflect the received light from the beam path when the received light propagates in a second direction different from the first direction, wherein

The first wavelength band includes wavelengths of the pre-pulse beam, and

the second wavelength band includes wavelengths of the primary beam.

2. The apparatus of claim 1, wherein the optical modulator comprises an acousto-optic modulator.

3. The apparatus of claim 2, wherein the apparatus further comprises a control system configured to provide a trigger signal to the acousto-optic modulator, and wherein the acousto-optic modulator deflects light away from the beam path and otherwise transmits light onto the beam path in response to receiving the trigger signal.

4. The apparatus of claim 1, further comprising a second optical modulator, wherein,

the second optical modulator is positioned between two of the dichroic optical elements, and

the second optical modulator is positioned to receive light transmitted by the two dichroic optical elements.

5. The apparatus of claim 4, wherein the optical modulator and the second optical modulator are located between the same two dichroic optical elements, and the second optical modulator is located on a second beam path different from the beam path.